Global issues, such as environmental problems and energy problems, have become more serious. Under such circumstances, solar cells using a clean and inexhaustible energy source have received a great deal of attention. Solar cells are generally used in the form of modules under outdoor conditions, for example, on the roofs of buildings.
Such a solar cell is illustrated in FIG. 1. The solar cell is fabricated by stacking a first sheet 4 for a sealing member on the surface of a front protective member 1, stacking a plurality of solar cell devices 3 on the sheet 4, stacking a second sheet 4 on the solar cell devices, and stacking a rear protective member 2 on the second sheet 4. The first sheet 4 is made of a thermal adhesive resin, and each of the protective members 1 and 2 is made of glass or a resin.
The laminate is fabricated by pressing the two protective members 1 and 2 together under heat and melting the two sheets 4 to integrate the protective members with the sheets. FIG. 2 illustrates a finished solar cell module fabricated by integrating the two sheets 4 in a molten state with the other members. The integrated sheets 4 form a sealing member 4′.
Each of the sheets 4 is often produced by extrusion or calendaring molding of a thermal adhesive resin sheet through a T die. The thermal adhesive resin sheet is stretched in the machine direction and exhibits high thermal shrinkage. For these reasons, the thermal adhesive resin sheet used as the sheet 4 suffers from severe shrinkage during thermal melt-bonding. This shrinkage causes misalignment in the position of the solar cell devices 3 or application of stress to the solar cell devices 3, leading to damage to the solar cell devices 3.
Bubbles containing harmful gases are generated in the course of stacking and laminating the sheets, the solar cell devices and the protective members. The bubbles can be removed by a vacuum process. The vacuum process for releasing the bubbles from the sheets is time-consuming, entailing an increase in the fabrication cost of the solar cell module.
Bubbles remaining unremoved from the solar cell module tend to contract at low temperature and expand at high temperature when the solar cell is exposed to outdoor conditions for a long time. This repeated contraction and expansion leads to the formation of cracks in the solar cell module and adversely affects the long-term durability and reliability of the product.